If a name is aliased by using keyword, and this name need be used as a forward declaration, which name should be used? The origin name or the new name? The name could refer to a plain data type, a class, or a namespace.
For example:
A.h:
using ID = int;
namespace AA {class C;}
using namespace BB = AA;
B.cpp
//here function f is implemented
#include "A.h"
B.h: here ID need a forward declaration
class ID;
namespace BB {class C;}
void f(ID i, BB::C c);
or we could only choose belowing one
//class int;
namespace AA {int C;}
void f(int i, AA::C c);
